Microelectronic, semiconductor, and other electronic components are often sealed in a cavity within protective packaging material, with lead wires extending from the circuitry to the exterior of the protective package for connection to other components. The protective package is intended to hold the circuitry in place and protect it against corrosion, oxidation, shock, handling, temperature, and other problems that can result in failure.
Although a number of different materials including plastic can be used, high-reliability devices often employ ceramic packaging. The ceramic packaging can provide a hermetic (air-tight) seal and superior heat dissipation. However, leaks, breaks, and other defects in the ceramic or other packaging may develop during manufacturing that affect the hermeticity of the seal and thereby threaten contamination and eventual malfunction of the circuitry within.
Many manufacturers and purchasers of high-reliability packages follow military defined specifications that prescribe hermeticity testing which includes gross leak testing and fine leak testing. Gross leaks are generally defined in the specifications as defects leaking at a rate of one-one hundred thousandth of a cubic centimeter (10-.sup.5 cc) per second or more, and fine leaks are generally defined as defects leaking at a rate as small as one-one hundred billionth of a cubic centimeter (10-.sup.11) per second. Problems unique to each leak rate dictate different testing methods for each.
Recognized test for gross leaks include the "weight gain method" and the "bubble method". Both methods involve holding the package under vacuum for approximately one hour and then attempting to backfill the cavity under pressure within an inert fluorocarbon bath, followed by removing the package from the bath, waiting a required amount of time for liquid on the exterior of the package to evaporate, and finally determining whether any fluorocarbon liquid was introduced into the cavity through a leak.
Under the weight gain method, the package is weighed before and after attempted backfilling, a change in weight being indicative of a leak.
Under the bubble method, the package is immersed in a liquid bath having a temperature above the boiling point of the liquid fluid introduced into the cavity which in turn causes detector-fluid vapor to evolve from the cavity ("outgassing"), bubbles rising from the package being indicative of a leak.
Both methods serve to determine whether fluorocarbon liquid was introduced into the cavity through a leak, but both require the time and careful attention of a trained operator. These methods are slow, costly, dependent upon human attention and judgment, and especially susceptible to operator error.
Therefore, it is desirable to have a superior method of gross leak detection in electronic packages.
It is particularly desirable to have a better method to detect outgassing of any detector-fluid vapor during hermeticity testing.
It is further desirable to avoid dependence on human attention and judgment and to have a reproducible and cost effective test suitable for quality control.
It is also desirable that the improved method be readily adaptable to automation.
And, it is desirable to avoid the cost and inconvenience of the large quantities of fluorocarbon fluid typically used in production testing by the bubble method.